In the title compound, {(C4H12N2)2[Li2(P6O18)]·4H2O}n, the phosphate ring anion, located around an inversion center, adopts a chair conformation. Adjacent P6O18 rings are linked via corner-sharing by LiO4 tetrahedra, generating anionic porous {[Li2(P6O18)]4-}n layers parallel to (101). The piperazine-1,4-diium cations occupy the pores and develop hydrogen bonds with the inorganic framework. An extensive network of N-HO and O-HO hydrogen-bonding interactions link the components into a three-dimensional network and additional stabilization is provided by weak C-HO hydrogen bonds.

The area of framework materials continues to be of interest not only because of
the wide variety of structures but also due to their potential applications in
the areas of catalysis, sorption and separation processes (Mahesh et
al., 2002) Natarajan, 2000). Much attention has been
devoted to the
synthesis of open-framework phosphates which exhibit a rich structural
diversity and have been widely studied as catalysts, ion-exchangers and as
positive electrode in the lithium and sodium batteries (Assani et al.,
2012). Within this family of compounds, the resulting anionic
frameworks,
generally constructed from PO4 tetrahedra that are vertex linked with MOn
polyhedra (with n = 4, 5 and 6), generate pores and channels offering suitable
environment to accommodate different other cations. The piperazine
(C4N2H10), which is a common heterocyclic nitrogen compound, has been
indicated as excellent template for preparing microporous materials (Xu et
al., 2007). The crystal structure reported here gives another
illustration
of this type of material. The corresponding compound,
(C4H12N2)2Li2P6O18.4H2O (I), is an organic-inorganic hybrid
built of two main cyclic components, C4H12N2 and P6O18 (Fig. 1). The
phosphoric rings are interconnected by the Li+ cations via LiO4
tetrahedra sharing corners to form a two-dimensional inorganic framework
extending along the (101) plane as shown in Fig. 2. The diprotonated
(C4H12N2)2+ cations are trapped within the 10-membered ring pore of
the layer, whereas the water molecules are located in the interlayer region
and are grafted onto the framework oxygen atoms through hydrogen bonds
(Fig. 3). The asymmetric unit of this atomic arrangement is built of one half
of the P6O18 ring lying on an inversion center (1/2, 1/2, 1/2), one Li+
cation, two water molecules and one piperazine-1,4-diium cation. The organic
and inorganic rings adopt a chair conformation with different geometrical
characteristics due to their different size and flexibility. However, the
P6O18 ring has (P–O and O–O) distances and (O–P–O, P–O–P and P–P–P
angles) comparable to those observed in other cyclohexaphosphates having the
same internal inversion symmetry (Abid et al., 2011; Amri et
al.,
2009; Marouani et al., 2010). The LiO4 tetrahedra is
slightly
distorted with Li–O distances ranging from 1.877 (4) to 1.969 (4) Å. The
smallest distance between two tetrahedral centers is 5.548 (2) Å. The organic
ring has for carbon atoms (C1, C2, C3 and C4) almost coplanar (r.m.s. deviation
from the mean plane = 0.014 Å) and N1 and N2 displaced from the plane by
0.672 (2) and -0.663 (2) Å, respectively. These characteristics do not differ
from those particular values observed in other compounds of the piperazinium
despite the different constraints of their solid states (Essid et al.,
2010).

Crystals of the title compound were prepared by adding dropwise and stirring an
ethanolic solution (5 mL) of piperazine (10 mmol) then an aquous solution (10 mL) of KOH (10 mmol) to an aqueous solution (10 mL) of cyclohexaphosphoric
acid (5 mmol). Colourless prismatic crystals were obtained after a slow
evaporation over a few days at ambient temperature. The cyclohexaphosphoric
acid H6P6O18,was produced from Li6P6O18.6H2O, prepared according
to the procedure of Schülke and Kayser (Schülke & Kayser, 1985),
through
an ion-exchange resin in H-state (Amberlite IR 120).

N and C-bound H atoms were positioned geometrically (N–H = 0.90 Å,
C–H = 0.97 Å) and allowed to ride on their parent atoms, with
Uiso(H) = 1.2 Ueq(C,N). The bond distances of O–H
and distance between two H atoms from each water molecules was restrained to
be 0.85 and 1.37 Å with the default deviation respectively and with
Uiso(H) = 1.5 Ueq (O).

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes)
are estimated using the full covariance matrix. The cell e.s.d.'s are taken
into account individually in the estimation of e.s.d.'s in distances, angles
and torsion angles; correlations between e.s.d.'s in cell parameters are only
used when they are defined by crystal symmetry. An approximate (isotropic)
treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s.
planes.

Refinement. Refinement of F2 against ALL reflections. The weighted R-factor
wR and goodness of fit S are based on F2, conventional
R-factors R are based on F, with F set to zero for
negative F2. The threshold expression of F2 >
σ(F2) is used only for calculating R-factors(gt) etc.
and is not relevant to the choice of reflections for refinement.
R-factors based on F2 are statistically about twice as large
as those based on F, and R- factors based on ALL data will be
even larger.